I mean I'm sure the people at Marvel know what they are doing, they're pretty much 6 for 6 with producing awesome comic book inspired movies in the MCU while added characters and keeping the fanbase interested. Janet in IM3 is either going to be like Widow in IM2 where she's a supporting character we don't know much about but you can tell she has more to come in the future, or she'll be like Hawkeye in Thor who's kinda just a throwaway character for people to be like: "wow cool it's hawkeye!". I'm assuming she's in more a Black Widow role since more will be coming from her, Hank, and Ultron in Phase2.. plus that scary Thanos fella in the background somewhere